U.S. patent application number 12/074651 was filed with the patent office on 2008-09-11 for plasma spraying for semiconductor grade silicon.
This patent application is currently assigned to Integrated Photovoltaics, Inc.. Invention is credited to James E. Boyle, Raanan Y. Zehavi.
Application Number | 20080220558 12/074651 |
Document ID | / |
Family ID | 39738631 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080220558 |
Kind Code |
A1 |
Zehavi; Raanan Y. ; et
al. |
September 11, 2008 |
Plasma spraying for semiconductor grade silicon
Abstract
A plasma spray gun configured to spray semiconductor grade
silicon to form semiconductor structures including p-n junctions
includes silicon parts such as the cathode or anode or other parts
facing the plasma or carrying the silicon powder having at least
surface portions formed of high purity silicon. The semiconductor
dopant may be included in the sprayed silicon.
Inventors: |
Zehavi; Raanan Y.;
(Sunnyvale, CA) ; Boyle; James E.; (Saratoga,
CA) |
Correspondence
Address: |
Law Offices of Charles Guenzer
2211 Park Boulevard, P.O. Box 60749
Palo Alto
CA
94306
US
|
Assignee: |
Integrated Photovoltaics,
Inc.
Sunnyvale
CA
|
Family ID: |
39738631 |
Appl. No.: |
12/074651 |
Filed: |
March 5, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60893684 |
Mar 8, 2007 |
|
|
|
Current U.S.
Class: |
438/57 ;
257/E21.092; 257/E31.012; 315/111.21 |
Current CPC
Class: |
Y02E 10/546 20130101;
H01L 31/182 20130101; Y02P 70/521 20151101; H01L 21/02532 20130101;
Y02P 70/50 20151101; H05H 1/42 20130101; H01L 21/02631 20130101;
C23C 4/134 20160101 |
Class at
Publication: |
438/57 ;
315/111.21; 257/E31.012 |
International
Class: |
H01J 7/24 20060101
H01J007/24; H01L 31/18 20060101 H01L031/18 |
Claims
1. A plasma gun for exciting a plasma in a stream of an arc gas, a
surface portion of at least one of parts of the gun facing the
plasma or a flow of powder into the gun consisting essentially of
silicon.
2. The plasma gun of claim 1, wherein the at least one part
includes at least one electrode of multiple electrodes of the
gun.
3. The plasma gun of claim 2, wherein one or more of the electrodes
are doped to be conductive.
4. The plasma gun of claim 2, having auxiliary heating means for
heating the one electrode to a temperature at which it can act as
an electrode.
5. The plasma gun of claim 1, wherein the at least one part
includes at least one powder injector for injecting powder into the
stream and having at least a surface portion facing a flow of the
powder consisting essentially of silicon.
6. A plasma spraying method, comprising: exciting a plasma in a
stream of an arc gas in a plasma gun having at least one electrode
having a surface portion facing the plasma consisting essentially
of silicon; injecting silicon powder into the stream having a metal
impurity level of less than 10 parts per million weight; and
directing the stream with the injected silicon to a substrate to
form a silicon layer thereupon.
8. The method of claim 6, wherein the silicon layer forms part of a
semiconductor device having a p-n junction.
9. The method of claim 8, wherein the semiconductor device
comprises a solar cell.
10. The method of claim 6, wherein the silicon powder consists of
particles 95% of which have diameters of less than 10
micrometers.
11. The method of claim 6, wherein chemical vapor deposition forms
the large-particle silicon powder or a larger body ground into the
suitably sized fine-particle size range of silicon powder.
12. The method of claim 6, further comprising the prior step of
heating the surface portion to be electrically conductive.
Description
RELATED APPLICATION
[0001] This application claims benefit of provisional application
60/893,684, filed Mar. 8, 2007.
FIELD OF THE INVENTION
[0002] The invention relates generally to plasma spraying. In
particular, the invention relates to plasma spraying in the course
of semiconductor fabrication.
BACKGROUND ART
[0003] Plasma spraying is a well established technology in which
powder of a selected material is entrained in a plasma-excited
stream of an arc gas directed at a substrate to be coated. The
powder is melted or vaporized within the plasma and coats the
substrate with a continuous layer of the material of the powder.
Usually the arc gas is inactive, such as argon, so only powder
material coats the substrate. Plasma spraying is particularly
useful for coating foreign substrates with a layer of a material
having a high melting point and which is difficult to machine, for
example, refractory metals. Suryanarayanan provides an overview of
plasma spraying in his text "Plasma Spraying: Theory and
Applications," World Scientific (1993), incorporated herein by
reference. Pawlowski provides another overview in his text "The
Science and Engineering of Thermal Spray Coatings," Wiley (1995),
also incorporated herein by reference.
[0004] Plasma spraying of silicon has been suggested for two
different application. Noguchi et al. in U.S. Pat. No. 5,211,76
disclose plasma spraying of a silicon adhesion layer in the
formation of a silicon solar cell. Such a solar cell may be
deposited on a low-cost substrate, whether glass, steel, or even
plastic. Akani et al. describe the semiconductor properties of
plasma sprayed silicon in "Influence of process parameters on the
elecgrical properties of plasm-sprayed silicon," Journal of Applied
Physics, vol. 60, no. 1, 1 Jul. 1986, pp. 457-459. Boyle et al. in
U.S. Pat. No. 7,074,693 disclose plasma spraying of a silicon
bonding layer bridging a seam between two silicon members to form a
structure used in semiconductor processing. Examples of such
structures are a tubular silicon oven liner and a silicon support
tower used in batch thermal processing.
[0005] To our knowledge, application of sprayed silicon to solar
cells has never been commercialized.
SUMMARY OF THE INVENTION
[0006] A plasma spray gun configured for spraying silicon includes
parts having at least surface portions composed of silicon.
Preferably, the silicon has an impurity level of heavy metals of
less than 1 parts per billion atomic.
[0007] The plasma gun of the invention may be used to spray
semiconductor grade silicon to form semiconductor structures
including, for example, a p-n junction. The sprayed silicon may be
doped to the respective semiconductor type. The silicon powder may
be obtained by jet milling in a jet mill with silicon walls
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a partially sectioned orthographic view of a
plasma spray gun to which the invention has been applied.
[0009] FIG. 2 is an orthographic view of an injector and injector
holder usable with the plasma spray gun of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] We believe that the plasma sprayed silicon used in any
application involving a semiconductor must be highly pure and free
of contaminants. We further believe that conventional plasma spray
guns and silicon powder used in plasma spraying silicon introduce
impurities in the sprayed film which deleteriously affect the
eventual product, whether it be the silicon solar cell or a silicon
integrated circuit thermally processed with fixture spray bonded
together. Suryanarayanan in the above cited text has disclosed how
the various metal impurity levels increase as silicon powder goes
through a plasma spray gun.
[0011] High-purity silicon powder can be obtained by the method
described by Zehavi et al. in U.S. patent application Ser. No.
11/782,201, filed Jul. 24, 2007. It involves jet milling of larger
granules of silicon grown by chemical vapor deposition in a jet
mill modified to incorporate some high-purity, semiconductor-grade
silicon parts, particularly the walls of the milling chamber and
other parts coming in contact with the powder or milling gas flow.
The granules can be either ground from fragments of an ingot of
virgin polysilicon (electronic grade silicon or EGS) otherwise used
as feedstock for Czochralski growth of wafers or be obtained from
MEMC Electronic Materials, Inc. of St. Louis, Mo. or Wacker of
Berghausen, Germany as directly grown from silane and hydrogen in a
fluidized bed reactor. Such material, if carefully selected has a
total transition metal impurity of less than 10 ppba (part per
billion atomic). We have achieved metal impurity levels in silicon
powder milled from larger CVD pellets of less than 10 parts per
million weight. We think the impurity levels can be further
reduced. Note that these impurity levels do not include the levels
of carbon, nitrogen, and oxygen, which are often in the ppm range
but have little effect on semiconductivity.
[0012] For feedstock in a plasma spray gun, the granules of the
silicon powder should have a size of few nanometers to hundreds of
micrometers though many spray processes are optimized for powder
sizes of 1 to 5 micrometers with at least 95% of the particles
having a diameter of less than 10 micrometers. The small particles
produce denser, higher-quality semiconductor films.
[0013] Conventional plasma spray guns can be retrofitted with one
or more silicon electrodes or other parts exposed to the plasma or
against which the powder may collide in order to reduce the
impurities introduced in the spray silicon from the electrodes and
parts. A plasma spray gun 10 illustrated in the partially sectioned
orthographic view of FIG. 1 is commercially available from Sulzer
Metco of Westbury, N.Y. as model F4-MB. It includes a housing 12
and a core 14 fixed inside the cover 12 and including a base
extending through the bottom of the housing 12. A cathode 16
includes a tip 20 both arranged generally circularly symmetric
about a gun axis. An anode 22 surrounds the tip 20 of the cathode
16 but is separated and electrically isolated from it by an annular
gap 24. Insulating spacers separate the cathode 16 and anode 22.
The anode 22 includes a nozzle 26 surrounding a tubular nozzle
liner 28 extending to the exterior of the gun 10 along the gun axis
along which the plasma beam travels. An inactive arc gas such as
argon and/or helium is supplied to the back of the gap 24 and flows
over the cathode tip 20 and out the nozzle 24.
[0014] The cathode 16 is negatively biased with respect to the
anode 22. For example, the anode 22 is grounded and a negative DC
voltage is applied to the cathode 16 of sufficient magnitude to
excite the argon into a plasma as it flows between the two
electrodes 16, 22. The plasma argon flows out of the gun 10 through
the nozzle 26 toward a substrate being spray coated as a
high-velocity beam having a velocity up to 3050 m/s.
[0015] The illustrated gun includes passages for cooling water
although radiative cooling though fins may be satisfactory.
[0016] A powder injector holder 30 is fixed to the gun 10 at the
outlet of the nozzle 26. As better illustrated in the orthographic
view of FIG. 2, it includes two stubs 32 for supporting two powder
injectors 34 with diametrically opposed injector tips 36 pointing
toward the middle of the plasma beam exiting the nozzle 26. The
mixing may be performed in a powder feeder, either the one
available from Sulzer Metco or other similar ones specially
designed for high purity. The carrier gas and entrained silicon
powder are fed to the back of the powder injectors 34 and injected
into the plasma beam through the tips 36. It is possible to drop
the silicon powder into the plasma beam without the use of a
carrier gas. The plasma beam quickly itself entrains the silicon
powder and vaporizes or at least melts it since the plasma gas
temperature may be as high as 18,000.degree. C. as the beam exits
the gun nozzle 26, far above the melting point of silicon of about
1410.degree. C. or its boiling point of 2450.degree. C. The gas
temperatures within the external plasma beam quickly decrease away
from the nozzle 26.
[0017] The vaporized or melted silicon entrained in the gun's
plasma beam strikes the substrate and is coated on it while the
argon diffuses away. The gun data sheet reports typical spray rates
of 50 to 80 g/min and deposition efficiencies of 50 to 80%.
[0018] Conventionally, the cathode 16, anode 20, and nozzle liner
28 have been composed of brass and perhaps including a tungsten
coating or insert. We think a better readily available metal for
coating or insert for silicon plasma spray guns is molybdenum. The
powder injectors have conventionally been composed of steel or
carbide. We believe that these gun parts are being partially eroded
during plasma spraying and the constituents are being coated
together with the silicon. Especially the negatively biased cathode
16 is subject to sputtering of positive argon ions in the plasma.
Heavy metal concentrations of greater than 1 ppma (parts per
million atomic) in silicon are sufficient to seriously degrade its
semiconductor characteristics. Copper in brass gun parts is
particularly deleterious.
[0019] The performance of the gun can be improved by changing the
composition of parts facing the plasma or carrying the silicon
powder entrained in the carrier gas to silicon, especially
high-purity silicon. That is, the cathode 16 and other degradable
parts or at least their plasma facing surfaces should consist
essentially of silicon having less than 1 parts per million atomic
(ppma) and preferably less than 0.1 ppma of metal impurities.
Silicon is available in purities of better than 1 ppba with
reference to heavy metals. The silicon may be monocrystalline, for
example, grown by the Czochralski method s used for semiconductor
wafers, or may be polycrystalline. Polycrystalline silicon may be
cast or also grown by the Czochralski method. A desirable form of
polycrystalline silicon is randomly oriented polycrstalline silicon
(ROPSi) grown by the Czochralski method using a randomly oriented
seed and thereafter machined to final product, as described by
Boyle et al. in U.S. patent application Ser. No. 11/328,438, filed
Jan. 9, 2006 and published as U.S. patent application publication
2006/0211218. Another advantageous form of polycrystalline silicon
is the previously described virgin polysilicon. Boyle et al.
describes the machining of this highly stressed material in U.S.
Pat. No. 6,617,225.
[0020] Powder purity is improved by assuring that the gas lines
supplying the feed carrier gas and arc gas and the feeder supplying
the powder to the feed supply gas do not substantially contaminate
the silicon powder.
[0021] For fabrication of a semiconductor junction by plasma
spraying, it is possible to control the doping of the sprayed
layers by varying the doping of the powder, as described by
Janowiecki et al. in U.S. Pat. No. 4,003,770 and by Gulko et al. in
U.S. Pat. No. 4,101,923. Neither reference describe how doped
silicon powder is obtained. We believe the powder can be doped in a
diffusion furnace using, for example, phosphine or diborane as
dopant gases to produce the selected conductivity type, as is
conventionally done for wafers. Alternatively, the Czochralski or
float zone silicon used in forming the powder may be grown with the
proper doping introduced in the melt. Silicon powders of different
doping types allow a siliconp-n junction to be fabricated perhaps
even using the same plasma spray gun. It is also possible to form a
p-i-n semiconductor structure, such as are favored for solar cells,
by spraying an intermediate layer of undoped silicon powder.
[0022] An alternative method to control the doping of the sprayed
silicon layers is to form parts of the plasma gun from doped
silicon. In particular, the cathode of the plasma gun is subject to
argon sputtering during the spraying operation. As a result, the
silicon of the cathode enters the plasma beam at a controlled rate.
Accordingly, if the silicon cathode is composed of n-doped or
p-doped silicon, the sprayed silicon layer will be similarly doped,
assuming that the silicon powder and other contaminants do not
counter dope. Bulk doped parts can be obtained by using Czochralski
or float zone silicon of the desired doping, as described above for
doped silicon powder.
[0023] One complication of a silicon cathode or anode is that both
electrodes need to be sufficiently electrically conductive to
excite and maintain the plasma. Very pure silicon is considered
resistive with a resistivity of, for example, greater than 10
ohm-centimeter. Several means may be employed to make the silicon
electrodes conductive.
[0024] The previously discussed doped silicon electrodes may have a
sufficient doping level, for example, resistivity less than 0.2
ohm-cm for either doping type to increase its resistivity even at
room temperature to acceptable levels. However, the concentration
of dopants in silicon is limited by the onset of segregation and at
this concentration limit the doped silicon has significantly less
electrical conductivity than a metal. Care must be taken to not
initiate filamentary currents and fracturing the silicon
electrodes.
[0025] Several other means rely on the fact that the electrical
conductivity of lightly doped and essentially undoped silicon rises
with temperature. Electrodes in plasma guns generally operate at
relatively high temperatures to the extent that cooling is
required. Accordingly, once an auxiliary source has heated the
silicon electrode to its high operational temperature, typically
about 600 to 700.degree. C., the auxiliary heating may be
removed.
[0026] One auxiliary heating means inductively couples RF energy
into the silicon electrodes by an RF coil or antenna positioned
outside the gun, similarly to the RF heating done in float zone
purification of silicon ingots.
[0027] The gun can include embedded resistive heaters in thermal
contact with the silicon electrodes.
[0028] Another auxiliary heating method initially passes a
flammable gas through the normal argon flow path in the gun and
ignites the gas to form a torch or flame adjacent the silicon
electrodes. Once the electrodes have reached the requisite
temperature, argon is substituted and power is applied to the
electrodes to excite and maintain the argon plasma. The impedance
of the electrode pair can be monitored during heating. The
flammable gas may a fuel such as oxygen in combination with
hydrogen, propane, or propylene, as described in Suryanarayanan's
text for high-velocity oxygenated fuel.
[0029] The invention is not limited to the described plasma spray
gun. The plasma can be excited by other means such as RF driven
electrodes or by an RF-powered inductive coil. The tube around
which the inductive coil is wrapped may be resistive, lowly doped
silicon of high purity. The powder can alternatively be injected
into the stream of the arc gas upstream or downstream from the
plasma source region, perhaps in the nozzle region, or in the
source region itself. Wire electrodes, for example, of silicon, may
be used.
[0030] The entire conventional gun part does not need to be
composed of silicon. The part can be redesigned to be composed of
silicon only in the portion facing the plasma or silicon powder
stream.
* * * * *